Traditionally, metal carbide/transition metal composites, such as, for example, zirconium carbide (ZrC) disbursed in a tungsten (W) matrix, have been fabricated by a hot pressing process. Mixtures of metal carbide phase and transition metal matrix phase powders are placed in a pressure vessel and are subjected to elevated pressures while heated to high temperatures, typically in an inert atmosphere or under vacuum. Typically, substantially dense composite bodies are formed by first measuring, mixing and blending the raw materials as powders. The blended powders are then loaded into a simple geometrical model, such as a graphite die, where the blended raw materials undergo heating and pressing simultaneously. The simultaneous application of heat and pressure provides sufficient urging forces to cause the powders to sinter and substantially completely densify. Although hot pressing is not required per se for the sintering of such composites, sintering without the application of elevated pressures results in weak bodies characterized by densities only about 90 percent of theoretical and having poor thermal and mechanical properties. Therefore, the densified bodies so produced are limited by the constraints of the hot pressing die to simple shapes and moderate sizes. Further, hot pressing techniques require expensive hot pressing facilities and provide a slow rate of production. Moreover, the bodies produced by hot pressing techniques are simple and unfinished, thus typically requiring further diamond machining in order to produce a finished end product. Such machining adds considerable time and financial cost.
In the hot pressing processes, the attendant high pressures are necessary to provide sufficient driving force for substantial densification to occur, since the mixed carbide and transition metal powders alone typically lack sufficient self-diffusion characteristics when heated to sintering temperatures. The use of high sintering pressures addresses this problem by providing an externally generated driving force to the system, but also adds complexity and cost to the fabrication of the composite bodies, since hot pressing requires expensive facilities and provide a slow rate of component production. Further, the application of high pressure adds inherent geometrical constraints that limit the bodies so formed to simple geometric shapes. The hot pressing technique is therefore limited to the fabrication of simple shapes of moderately sized parts. Due to the very high melting temperature of many desirable matrix transition metals, such as metallic tungsten, and their low self-diffusion coefficient, the highest density obtained by hot pressing is typically less than 90% of theoretical density.
Recently, an alternate fabrication technique using displacive compensation of porosity (DCP) has been used to fabricate ZrC/W composites, in which a low melting point metal alloy (herein, Zr2Cu) is infiltrated into, and reacted with, a porous tungsten carbide (WC) preform at elevated temperatures, such as the range 1200-1300° C. for an extended soak period, such as 8 hours.
Although the DCP method lends itself to lower pressure processing and thus may be used to produce relatively large, complex shaped parts, it still suffers from several drawbacks. For example, the DCP method remains a two step process, with the first step being formation of a WC preform and the second step requiring the need for liquid metal infiltration followed by a reaction. Further, the pores produced in the WC preform, as required by the DCP method, cannot be completely eliminated after the infiltration and reaction step. Typically, 5% porosity remains in the final products of ZrC/W composites produced by the DCP method. In addition, metallic copper-rich phases, typically undesirable impurities having a relatively low melting point (≦1083° C.), cannot be completely eliminated during the DCP process, and thus remain as impurities. Moreover, the porosity and pore size distribution of the WC preforms cannot be precisely controlled. Therefore, the composition and microstructure of the final products are not reproducible or reliable, as W, ZrC, residual WC, Zr2Cu and porosity vary from part to part.
Thus, there remains a need for a simple, quick and low-cost means of fabricating and sintering metal carbide/transition metal matrix composite bodies, such as ZrC/W, having complex shapes at ambient pressures. The present novel technology addresses this need.